1. Field of the Invention
The present invention relates to preventing copying of digital data and more specifically to a system and method of retrieving an embedded watermark in a signal.
2. Discussion of Related Art
Digital Watermarking offers means to embed some additional hidden data into a host audiovisual signal in such a way that the resulting watermarked signal and the host signal are perceptually identical. Although a wide range of applications can benefit from this technology, watermarking methods have drawn much attention recently due to the rapid development of intellectual property rights protection issues. A typical watermarking algorithm embeds a watermark by adding noise patterns or echos to an original audiovisual signal such that the watermark is not perceptible but can be retrieved by using a correlation type of methods. In order to make these methods more robust in retrieval and pirate attacks, a stronger noise pattern or large echo has to be used. Unfortunately, the stronger noise pattern or large echo causes audible distortion in the resulting watermarked signal as well, which is not acceptable. Therefore, this tradeoff limits the robustness of these methods and makes them sensitive to other noises and distortions generated in the process following the watermarking operation, such as coding.
Some known methods may exploit the long- or short-term, temporal or spectral masking effects of the Human Auditory System (“HAS”). Literature such as W. Yost's “Fundamentals of Hearing, an Introduction” (Academic Press, New York) describe the HAS. However, since most modern audio compression algorithms also take full advantage of these same characteristics, those perceptually shaped watermarks (noise patterns or echos) may in fact be damaged by an advanced perceptual coder or at least their margins of exploiting masking effects may become limited.
Most watermarking methods available today are also called “blind” watermarking which means that the embedded watermark can be retrieved from the watermarked signal without requiring access to the unwatermarked original. This convenience makes them useful for carrying descriptive information associated with the actual audio contents, such as title, composer and players etc. However, since they are usually vulnerable to attacks as explained above, they are not good candidates for intellectual property protection.
What is needed in the art is a system and method for covert (or non-blind) digital audio watermarking.